There are several drivers with specialized engagement surfaces in prevalent use for industrial and domestic purposes. These drivers are, in general, only useable with screws having recesses which match the drivers. The manufacturing processes for these screws insure a proper fit with efficient transmission of torque from the driver to the screws. A common example of such drivers is the Philips head screw driver with its familiar cross wing configuration. A fastener of this type is disclosed in U.S. Pat. No. 2,046,839 and the disclosure of this patent is incorporated herein by reference. Fasteners of this type are in widespread use.
Although PHILLIPS head screw are used with great success, a common complaint with regard to such fasteners is that the driver tends to slip out of the recess, when the application of high torque is required. This so-called “cam out” occurs because the driving surfaces of the PHILLIPS head fastener taper at an included angle of 8°-27°. In order to overcome this inconvenience and to provide a fastener which provides a high torque capability, the driving surface was constructed more vertical as described in U.S. Pat. No. 2,474,994. The disclosure of the '994 patent is also incorporated herein by reference. Fasteners of this type are available from licensees of Phillips Screw Company and sold under the trademark POZIDRIV.
Another type of driver referred to as the “square drive” is designed to fit the Robertson square drive recess and is also in common use.
It is a purpose of this invention to provide a recess which effectively accommodates any of the above drivers and to provide an optimum driver adapted especially for use with this unique recess.
To better understand the invention of this application, it is beneficial to consider the basic process of manufacturing the drivers and mating fastener heads of the above referenced fasteners. It is key in the process to construct, to close tolerances, a die which is used to cold form drivers and punches by pressing. Punches are tools used to cold form the recess in the fastener head. Since each element is used progressively to obtain the final product, it should be realized that errors become compounded throughout the process.
A master tool having all of the attributes of the punch and driver is constructed as a first step. This tool is generally machined from an appropriate metal blank using an abrasive wheel. In the case of the PHILLIPS cross head tool, this requires four passes, between which the tool is rotated 90°. The tool is held at angle to the path of the wheel to construct grooves in the master tool that become more shallow as the machining stroke progresses from the tip to the shank of the master tool blank. The die is formed using the master tool by hobbing or pressing the master tool into die stock. This may take several strokes to obtain the fully formed die. The die is then used to cold form a punch which is in turn used as the tool to cold form a recess in the head of the fastener.
Drivers are constructed in a manner similar to the punches and are identical in shape to the punch and master tool.
The manufacture of the PHILLIPS cross head master is relatively straight forward because the driving surfaces are symmetrical. This is not the case with respect to the manufacture of the vertical driving surfaces of the modified cross head fastener of the '994 patent. In this instance, it has been found that to obtain an accurate relationship with respect to the driving surfaces of a wing of the recess and that of the driver, it is necessary to construct the forward driving face vertical and the reverse driving face at a small taper. The planes of these tapered surfaces, if extended, would intersect at an included angle of 2.5° or looking at it another way the reverse driving surface is tapered at an angle of 2.5° to the forward driving surface. This provides the needed relief from cam out in the forward direction where it is more problematic.
To accomplish this according to the above process, the master tool blank is machined using eight strokes, between which, the blank is rotated and the angle of the blank with respect to the wheel path is also adjusted. This involves the machining of compound angles, i.e., angles which are dependent on other angles and is extremely complex and difficult to control to the required tolerances. The machining of the master tools for such asymmetrical cross head screws is an acknowledged challenge.
Although attempts have been made to combine the Robertson square drive with a PHILLIPS type drive, see U.S. Pat. Nos. 5,358,368 and 5,020,954, these have not been well received and do not accommodate the use of an asymmetrical driver. U.S. Pat. No. 2,082,748 discloses a fastener that combines a Robertson recess with a PHILLIPS type recess and a slot type recess. It appears that such attempts have generally compromised the advantage of one type of recess or the other.
It is a purpose of this invention to provide a recess for a fastener which will accommodate a square driver, and symmetrical and asymmetrical cross bladed type drivers. This recess therefore, will accommodate the wide variety of fastener drivers currently in use. It is a purpose of this invention to provide such a recess while avoiding the complex manufacturing requirements of the asymmetrical type cross head and without compromising the beneficial features of the common driver configurations. It is also a purpose to provide a unique driver to take and advantage of all of the unique features of the new recess.